Spark plug with the insulator tip coated with spinels of aluminum and cobalt oxides



Jan. 23, 1968 H. LINSTEDT 3,365,605

' SPARK PLUG WITH THE INSULATOR TIP COATED WITH SP INELS CF ALUMINUM AND COBALT OXIDES Filed May 25, 1965 United States Patent Ofiice fih fih Patented Jan. 23, 1968 3,365,605 SPARK PLUG WITH THE INSULATOR TIP COATED WITH SPRNELS F ALUMINUM AND CGBALT OXEBES Hans Linstedt, Stuttgart, Germany, assignor to Robert Bosch G.m.b.H., Stuttgart, Germany Filed May 25, 1965, Ser. No. 458,594 Claims priority, application Germany, June 5, 1964, B 77,101 7 Ciairns. (Cl. 313-137) The present invention relates to a spark plug and, more particularly, to a spark plug for internal combustion machines. The present invention is particularly useful with respect to spark plugs which comprise an insulating body fixed in the spark plug housing and formed with an axial bore accommodating the center electrode. The insulator generally consists essentially of aluminum oxide and is so shaped and dimensioned that at its operating end, in the vicinity of the spark gap the end portion of the insulator will during operation reach an elevated temperature within a certain operating range, which temperature is determined by the thermal characteristics of the spark plug.

The thermal characteristics of the spark plug, with respect to all portions thereof which are heated during operation of the spark plug, should be such that the operating end portion of the insulating body of the spark plug which is exposed to the combustion gases should have a minimum temperature which should not be less than 500 C. for any prolonged period of time, so that combustion residues which may attach themselves to the operating end of the insulating body, particularly soot or carbonized fuel and oil will not be capable to form an e ectrically conductive bypass to the spark gap. At temperatures of at least about 500 C., such carbonaceous deposits at the operating end portion of the insulating body will be subjected to combustion without leaving a solid residue.

On the other hand, the spark plug should not reach a temperature of above about 1000 C., preferably be tween about 800 and 900 C., in order to avoid premature destruction of the spark plug. This upper temperature limit is determined by the thermal characteristic of the metals of which the electrodes of the spark plug are formed, particularly with respect to resistance to scaling and thermal decomposition. Even spark plugs with platinum electrode preferably should not be heated to a temperature higher than 900 0, since otherwise the surface of the operating end of the insulator will be incrustated with fuel residues which will no longer be burned off. Although these incrustations of fuel residues which form at higher temperatures are not electricall conductive, the same are harmful because they possess a considerable absorptive capacity and thus, upon starting of the internal combustion machine will absorp the fuel and thereby obtain a sufficiently high degree of electric conductivity that the spark gap with its high electric resistance could become ineffective due to a secondary electric contact between the electrodes formed by the fuel-containing incrustations on the insulating body.

It is thus important that the operating temperature of the spark plug will be maintained between an upper and a lower limit. However, it happens sometimes that internal combustion engines, particularly automobi e engines are to be operated for prolonged periods of time at a load factor higher or lower than that originally contemplated. This, for instance, is the case when the automobile is operated for a prolonged period of time at a very low load factor in congested city trafiic, or when in long distance travel at high speed the contemplated upper load factor is reached or exceeded. For these reasons, it is not possible to provide a spark plug which possesses thermal characteristics which are a priori fully suitable for all operating conditions to which the internal combustion engine and thus the spark plug may be exposed. In view thereof, it has been attempted to increase the temperature range within which the spark plug will operate effectively and without causing difficulties.

It has been proposed to so design spark plugs and to so choose the materials of which the same are produced, that the thermal elasticity will be as great as possible, in other words, so that the spark plug will be capable of effective functioning within a relatively wide temperature range. For this purpose, it has been suggested, for instance, to space the center electrode through which heat is conducted away from the combustion chamber, from the operative end portion of the insulating body. Thereby it is supposed to be accomplished that the operating temperature of the initially cold spark plug increases as quickly as possible and, on the other hand, it has been proposed to use electrode metals of particularly high heat conductivity in order to prevent overheating of the electrode tip, and also to cool or prevent further heating of the operating end portion of the insulating body, particularly at relatively high operating temperatures. In such a case, heat radiation from the insulator which increases with the fourth power of 10 of the absolute temperature will convey considerable amounts of heat through the separating gap to the center electrode, which heat will then be conducted away by the center electrode. However, these improvements of spark plugs do not suflice to obtain satisfactory results under all possible conditions, for instance in the case of spark plugs which are incorporated in racing engines where the operating conditions are particularly critical due to the high degree of compression. In such cases it is particularly important to keep the temperature of the spark plug at high load factor operation below the permissible maximum temperature so that knocking of the fuel and undesirable and harmful premature ignition will be avoided.

It is therefore an object of the present invention to provide a spark plug which will to a very great extent overcome the above-discussed difficulties and disadvantages.

It is a further object of the present invention to provide a spark plug wherein the operating end portion of the insulating body is capable of very considerable heat radiation particularly at elevated temperatures and in the vicinity of the upper limit of the above-discussed temperature range.

It is thus an additional object of the present invention to provide a spark plug wherein the desired upper limit of the operating temperature will not be exceeded even under operating conditions which deviate greatly from the originally contemplated or standard operating conditions.

Other objects and advantages of the present invention will become apparent from a further reading of the de scription and of the appended claims.

With the above and other objects in view, the present invention contemplates in a spark plug, in combination, first and second electrode means having end portions respectively located adjacent but spaced from each other, the end portions forming a spark gap therebetween, and an insulating member having an end portion located in the vicinity of the spark gap-forming end portions of the electrode means so that upon operation of the spark plug in an internal combustion machine the outer surface of the end portion of the insulating member will be exposed to hot combustion gases, the outer surface of the insulating member consisting of a material having within a tem- '3 perature range of between 500 and 1000 C. and within a wave length range of between 1 and 7 microns an emissivity of at least 0.7.

The present invention also proposes a spark plug, comprising, in combination, a housing formed with an axial bore therethrough, an elongated tubular insulating member extending fluid-tightly sealed through the bore of the housing, central electrode means extending fluid-tightly sealed through the tubular insulating member and having a free end projecting beyond one end portion of the tubular insulating member, a second electrode fixed to the housing and having a portion opposite and spaced rom the free end of the central electrode means to define a spark gap therewith, the one end portion of the tubular insulating member being subjected at the outer surface portion thereof, upon operation of the spark plug in an internal combustion machine, to contact with hot combustion gases, the outer surface portion of the one end portion of the tubular insulating member being formed of a layer of a material consisting essentially of spinels of aluminum oxide and cobalt oxide and having within a range of between 500 and 1000 C. and within a wave length range of between 1 and 7 microns an emissivity of at least 0.7, said layer having a thickness of between about 0.02 mm. and 0.1 mm.

Thus, according to the present invention, the surface portion of the insulating body of the spark plug which during operation will be exposed to hot combustion gases consists of, or contains, materials having an emissivity, particularly within the wave length range of between about 1 and 7 microns, which approaches the emissivity of a black body. The emissivity of this surface portion of the insulating body of the spark plug will be at least about equal to 0.7 of the radiation emitted within the same temperature, namely between 500 and 1000 C. and in the same wave length range, namely between 1 and 7 microns, by a complete radiator or black body. Since heat radiation increases with the fourth power of the absolute temperature, it is possible in this manner to obtain a spark plug insulator which, like all other ceramic insulators has at increasing temperatures a decreasing heat conductivity, which, however, with increasing temperatures, particularly in the vicinity of the upper limit of the above-described temperature range, will give up increasing amounts of heat by radiation. This phenomenon, namely increased heat radiation at higher temperatures can be found only to a very slight degree in the conventional light or only slightly colored insulating materials and seems to have found no consideration up to now.

In this connection, the insulating bodies of sintered alumina and the like show a behavior which is very different from that of glass, partly because glass generally is transparent and thus does not absorb significant or considerable amounts of radiation energy, while the material of which the insulating bodies of spark plugs are made have a very high absorption coefiicient which primarily impedes the inner radiation of the major portion of the molecules which are located below the surface layer. This is contrary or opposite to the conditions prevailing with respect to glass and this is also an explanation for the fact that hot glass cools much more quickly than corresponding bodies of ceramic material with comparable heat conductivity. The importance of this additional inner radiation which must not be mixed up with external radiation is known also in connection with materials other than glass. For instance, the Welsbach incandescent mantles of gas lights were so chosen that in addition to optimum yield of visible light radiation, the red, infrared and hot radiation components were sup pressed and radiation of shorter wave length in the direction towards violet rays was favored.

According to the present invention the reverse selection is carried out, namely to provide the surface of the operating portion of the spark plug insulating body with A a preferred capability of radiating red and infrared rays so that heat will be dissipated and thus adverse and exces sive rising of the temperature of the insulating body which could be harmful to the electrodes and could cause incrustations, will be prevented. For the purpose of the present invention particularly metal oxides such as cobalt oxide or mixtures of oxides are suitable which in the range of infrared radiation show a behavior which approaches that of the theoretical black body. It is not required that the heat radiation effect of the heat exposed surface portion of the insulator is achieved by forming such surface portions of a material which has a black appearance and thus emphasizes the visible spectrum. What is important, is that the material of the surface portion of the operating end of the insulating body will favor the relatively long wave heat radiation of rays having a wave length of at least about 1 micron. Thus, it is possible to use for the heat radiating surface portion layers which have a light appearance, for instance cerium oxide, since within the invisible range of heat rays, cerium oxide approaches the radiation characteristics of a black body.

In the last mentioned case, it is possible to achieve the advantages of the present invention with respect to dissipation of heat from the insulating body by radiation, with visual surface characteristics such that the surface of the operating end portion of the insulator will not be of black but rather of light appearance, so that it will be possible by visible inspection to determine whether any dark depositions or incrustations have formed on the surface portion of the insulating body.

Thus, the surface of the operative end portion of the spark plug insulator will approach, in accordance with the present invention, within the temperature range of between about 500 and 1000 C. and for a wave length range of between about 1 and 7 microns the characteristics of the theoretical black body and will have anemissivity of at least about 0.7 and preferably higher. This surface portion of the insulator will not only radiate heat outwardly towards the housing of the spark plug but, surprisingly, will also emit a considerable amount of radiation energy towards the center electrode, particularly at the end portion thereof which forms part of the spark gap and in the vicinity of which the insulator of the spark plug is relatively thin and therefore located closely adjacent to the center electrode. This ability for inner radiation, i.e., for heat radiation from the heat exposed surface through the relatively thin insulating body towards the opposite surface which faces the center electrode, or which also can be observed in known manner with respect to visible light by the transparency of thin porcelain bodies, will lead, particularly at high temperatures, to the passage of considerable amounts of heat radiation energy from the surface portion of high emissivity towards the interior of the insulator structure. This passage of heat energy from the surface into the interior of the insulating body does not take place to an appreciably extent in the case of conventional insulating bodies the surface of which does not possess the high emissivity within the described temperature and wave length ranges. The amount of heat which is thus passed through the thin operating end portion of the insulating body to the center electrode will increase with increase in the temperature difference between the surface of high emissivity of the insulating body, which surface is exposed to the hot combustion gases, and the temperature of the center electrode. Since the center electrode per se is also capable, like all metals, to radiate heat at a high rate, while ceramic insulating materials basically possess a high absorption coefficient for heat radiation, the following conditions will prevail:

Generally, in the case of conventional spark plugs heat radiation takes place from the center electrode toward the insulating body, while in contrast thereto, the insulating body according to the present invention due to the fact that it approaches the characteristics of the black body in its outer surface portion will cause radiation through the insulating body towards the center electrode, as long as the temperature at the outer surface of high emissivity and inner radiation of the insulating body will be higher than the temperature of the portion of the center electrode which is surrounded by the operating end poi ion of the insulating body.

Due to the fact that the insulating body of the spark plug is not formed of a homogeneous mass like a fused mass but is sintered body, the absorption coefl'icient a, which is determinative for the radiation conductivity, i.e. the inner radiation which is superposed upon the heat conductivity, is considerably greater than the absorption coefiicient :1 of a fused homogeneous insulating material.

On the other hand, it has been found that the absorption coefiicient which is measured by the deviation of visible optical parallel light rays and which in accordance with conventional measuring methods is a greater by several powers of 10, cannot be used as a correct measure for the inner heat radiation which is utilized according to the present invention.

In the optical experiment with visible light, all of the initially parallel light rays which are deflected are considered as absorbed, while in the case of heat radiation, the preponderant portion of the deflected rays are subsequently again deflected, usually several times, and thus pass through the molecular combination of the sintered bodies, so to say, along a meandering or zig-zag path, and thus continue to participate in the total heat radiation.

It can be calculated in the case of insulating bodies of aluminum oxide, from the diffused reflzxivity of the same, that the heat absorption of such a sintered body is very close to the heat absorption of a homogeneous or fused mass and these calculated values are confirmed by practical experience. It is known that the absorption coeflicient a for a homogeneous material equals about 0.1 cm. while the optical absorption coefiicient for the sintered material, determined with parallel light rays, equals about 100 cmf On the other hand, the heat radiation absorption coefiicient can be calculated from the diffuse reflexivity as about 0.5 cmf This is important in order to understand that the effect which is achieved according to the present invention cannot be understood by optical consideration with respect to the behavior of visible light rays, but only by taking into consideration the very considerable difference between the deflection of optical visible light as compared with the deflection of heat radiation. Thus, the insulating body of the spark plug may be considered more or less like a luminous glowing body which radiates, inwardly from its surface layer of high emissivity, heat rays through the interior of the insulating body towards the relatively closely adjacent opposite surface thereof which faces the center electrode so that the center electrode of lower temperature is exposed to heat radiation from the adjacent face of the insulating body.

It has been found that the temperature of a given conventional spark plug in the hottest zone of the operating end portion of the insulating body will under certain operating conditions reach a value of 880 C., :15 C. By burning cobalt oxide into the surface portion of the operating end of the insulating body of the spark plug which is exposed to the hot combustion gases, it has been found that under similar operating conditions, the temperature of the hottest portion of the operating end of the insulating body of the spark plug will be considerably lower, namely only between 820 and 840 C. This is achieved by the greater emissivity of the cobalt oxide of the heat exposed surface portion which approaches (but of course does not reach) the emissivity of a black body. This reduction in the temperature of the hottest portion of the insulating body of the spark plug is particularly marked in the vicinity of the upper limit of the temperature range during operation of the spark plug. At or in the vicinity of the lower limit of the temperature range, i.e. at operating temperatures of about 500 C., the reduction in the temperature of the hottest portion of the insulating body, which is achieved by increasing the emissivity of the surface portion of the insulating body which is exposed to the hot combustion gases, will amount only to a few degrees C. It is achieved thereby that the range of suitable or useful thermal characteristics of the spark plug which depends primarily on the characteristics of the material of the insulating body and the shape and dimensions of the spark plug will be increased by about 60 C. It is found thereby that the heat radiation or emission of the surface portion of the insulating body which, in accordance with the present invention, is of high emissivity will increase within the upper range of operating temperature to nearly 3.5 times the heat radiation of a similar spark plug insulating body with an emitting surface of the conventional low emissivity.

Furthermore, the heat loss by interior radiation from the surface portion of high emissivity through the sinter structure of the insulator body which is transparent for heat radiation, will be about ten times that which would occur in the case of a similar insulator body of a spark plug in which, however, the surface portion which is exposed to the hot combustion gases is not of high emissivity. Thus, depending on the specific sintering characteristics, i.e. depending on grain size and absorption coeflicient a, and on the emissivity e of the entire sintered aluminum oxide or the like mass, the inner radiation will at operating temperatures of about 900 C. amount to about 25-30% of the amount of heat which is withdrawn from the heated surface by the heat conductivity of the aluminum oxide or the like. In the case of conventional insulating bodies, which do not possess a heat exposed surface portion of such high emissivity, the inner radiation of heat amounts to only a few percent.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which the drawing is an elevational cross sectional view through a spark plug in accordance with the present invention.

Referring now to the drawing, it will be seen that the spark plug consists essentially of an insulator 2 formed of sintered aluminum oxide (A1 0 which is gas-tightly fixed in spark plug housing 3 by means of sealing rings or gaskets 4 and 5.

Insulating body or insulator 2 is formed with a bore and surrounds center electrode terminal portion 6 which is connected to a source of electric current, and center electrode terminal portion 7 which teminates in an end portion forming a spark gap with electrode 9. The adja cent end portions of center electrode portions 6 and 7, located within the bore in insulator 2 are embedded and thus connected by means of a fused plug consisting of glass and electrically conductive material incorporated therein.

The operating end portion 10 of insulator 2 which is of light, substantially white color and which consists also of aluminum oxide is provided at the portion of its surface which is exposed to the hot combustion gases with a layer formed by the application of cobalt oxide which together with the aluminum oxide of the insulating body forms a layer 11 which consists of spinels Al O +CoO. This surface layer 11 will be of bluish-black color and will act within the infrared wave length range of between about 1 and 7 microns somewhat similar to the theoretical black body. The emissivity e of this layer is about 0.75. The thickness of the layer 11 of high emissivity at temperatures of between about 500 and 1000 C. and with respect to a wave length range of between about 1 and 7 microns should be equal to between about 2 and 3 times the reciprocal absorption coefficient of the layer. In the case of spin ls of aluminum and cobalt oxide the minimum thickness of the layer should be 0.02 min. and result obtained will improve with increase in the thickness of the layer up to about a maximum thickness of 0.1 mm. Increasing the thickness of the layer 11 to more than about 0.1 mm. does not result in a marked improvement of the outer and inner heat radiation.

In the presently described example, layer 11 is formed of spinels of aluminum oxide and cobalt oxide. However, this layer may be formed of any suitable material having an emissivity Within a temperature range of between about 500 and 1000" C. and for wave lengths of between about 1 and 7 microns which will be equal to at least about 0.7, and preferably higher, of the radiation which would be emitted by a complete radiatior or black body at the same temperature and under similar conditions, and which is suitable for forming the surface layer 11 of the insulating body 2, will firmly adhere to the insulating body 2 and will not react with the fuel or hot combustion gases to which it is exposed upon operation of the spark plugs.

A layer 11 consisting of spinels of aluminum oxide and cobalt oxide may be formed by applying to insulating body 2 by imersing, brushing or in any other suitable manner a saturated aqueous solution of cobalt nitrate, i.e. of Co(NO '6I-I O crystals dissolved in water. This solution must be applied at least to the portion of insulating body 2 at which layer 11 is to be formed. The solution may be applied to the completed insulating body consisting of sintered A1 or to the pressed body formed of alumina which is still to be burned in order to produce the sintered insulating body.

Thereafter, the thus applied aqueous solution of cobalt nitrate is dried with warm air and this process of applying solution and drying of the same is repeated several times until a crystalline layer having a thickness of about 0.08 mm. has been formed.

Thereafter, the insulating body is burned at 1600 C. Thereby C00 is formed which then combines with the A1 0 of the insulating body under formation of spinels while nitrogen oxides, predominantly N 0 escape in gaseous form.

Other materials which possess within the desired temperature and wave length range a high emissivity such as about 0.9 based on an emissivity of 1.0 of a complete radiator or black body, are described in the Research Report AD 272 614 by W. R. Wade and W. S. Stemps, entitled Total Emittance of Several Refractory Oxides, Cements and Ceramics for Temperatures from 600 Degrees F. to 2000 Degrees F., published by the Office of Technical Services, Washington, DC.

The heat disposal or loss of portion of insulator 2 is calculated in Table I for:

(A) An insulating body having an emissivity e of 0.30 and (B) An insulating body including a layer 11 having an emissivity of a 0.90. The temperature and amount of heat which is withdrawn is shown for three different sets of operating temperatures with respect to three points of the spark plug, namely at the point of the spark plug housing which is indicated in the drawing by A, at the gasket B, and at the point indicated by C of the end portion of the insulating body which is exposed to the hot combustion gases.

The emissivity e is based on an emissivity of 1.0 of the ideal black body which according to the StefamBoltzmann law radiates energy acording to the formula E=s.l wherein .r=5.68 lO W/cm. x degree and T :is the absolute temperature in Kelvin. The comparison values given in Table I show also the withdrawal of heat by heat conduction, as well as the sum total of heat conduction and outer heat radiation.

TABLE I OPERATING TEMPERATURES At spark plug housin C.) (A) 300 300 300 At insulatorbody adj cent gasket 5 C.) (B) 350 400 450 At tip of insulating body exposed to hot cornbustion gases C.) (O) 600 800 1,000

HEAT LOSS OR WITHDRAWAL By heat conduction (oak/sec.) 1. 7 2. 5 3. 2 By heat radiation of insulating body having emissivity of 0.30 (oak/sec.) 0.21 0. 48 0. 93

Total (cal/sec.) 1. 91 2. 98 4. 13

By heat conduction (oak/sea). 1.7 2. 5 3.2 By heat radiation of insulating body including layer 11 having an emissivity of 0.90 (cal./ sec.) 0.6 1. 42 2. 76 Total (cal/sec.) .Q- 2.3 3. 92 5. 96

Table I shows that the radiation of the insulating body of the spark plug which includes layer 11 of high emissivity is under all operating conditions about three times that of the heat radiation of the conventional spark plug in which the insulating body is formed without a layer 11 of high emissivity.

Furthermore, due to the proportionally greater increase of the absolute radiation value at higher operating temperatures, the combined heat loss or heat withdrawal by heat conductivity and heat radiation of the spark plug of the present invention which includes layer 11 will at low temperatures be greater by only about 20%, at medium temperatures by about 30% and at the particularly important temperatures in the vicinity of about 1000 C. by nearly 50% than the total heat loss of the conventional spark plug.

Table II will serve to show the improvement in the inner heat radiation from the surface portion of the insulator which is exposed to the hot combustion gases through the sinter structure of the insulator to the portion of the insulator contacting gaskets 5 and to the considerably cooler center electrode 7, which improvement is achieved by providing a layer 11 of high emissivity, assumed in the present case to be 0.9.

Since the inner radiation through the sinter body of the insulator and the heat conduction through the same are superposed upon each other it does not seem possible to separate the values for inner heat radiation and heat conduction in an experimental manner. In order to avoid the difficulties involved in calculating these conditions for a spark plug, the calculations summarized in Table II are based on a simplified model, namely a sintered aluminum oxide rod having a length of 1 cm., and for a constant temperature of 900 (3., however, for two difierent sintered masses which are defined by the equations 2r/d=l.6 cm. and 2r/d'=0.8 cmf In these equations r denotes the reflectivity and d is the grain size, whereby d is twice as big as d. These values correspond substantially to the conditions prevailing in conventional spark plugs wherein the value for 2r/d generally equals 1 cm.- :30%.

The heat transfer by internal or inner radiation of the model body was calculated for varying thicknesses D based on the following parameters:

a =O.1 cm. =a'bsorption coelficient of solid A1 0 A:0.5 cm. =absorption coefiicient of sinter-A1 0 with a sintering state of 2r/d=l.6

A'=0.36 cm.- =absorption coeflicient of sinter-A1 0 with a sintering state of 2r/d=0.8,

in accordance with the approximation formula:

with the values Zr/c 1.6, and 2r/d=0.8 for both sintering states.

TABLE II.HEAI REMOVAL BY INNER HEAT RADIATION OF MODEL BODIES OF VARYING THICKNESSES D AT A Table II shows that both models of sintered aluminum oxide bodies will have an inner radiation of only up to 4% with respect to the heat conductivity when the emissivity of the end face of the model body is relatively low, such as 0.30, while in the case of sintered model bodies with an absorption coefiicient A'=0.5 emfand an end face with an emissivity of 0.90 an inner radiation of between 12 and 22% occurs. In a sintered body with the more favorable absorption coefiicient A=0.36, the inner radiation may rise up to a value equal to 30% of the heat conductivity. At temperatures of about 500- C., the inner radiation will be insignificantly small so that the inner radiation values of 22% or 30% which are found at temperatures of 900 C. are substantially fully effective for improving the operative heat range of the spark plug by providing at the operating end of the insulating body a layer 11 of high emissivity.

Since at 500 C. heat conductivity is greater and inner heat radiation is much smaller than at 900 C., the spark plug according to the present invention will show at the lower limit of the operative temperature range of about 500 C. a heat loss by radiation and conductivity which will be quite similar to that of the conventional spark plug, while in the upper temperature range, for instance at about 900 C., in addition to the favorable results described in Table I, also a highly significant increase in the inner heat radiation, i.e. in the radiation of heat from layer 11 into and through the underlying sintered body, will take place.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of spark plugs differing from the types described above.

While the invention has been illustrated and described as embodied in the incorporation of a surface layer of high heat radiation or emissivity at the operating end of the insulating body of a spark plug, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A spark plug, comprising, in combination, a housing formed with an axial bore therethrough; an elongated tubular insulating member extending fluid-tightly sealed through said bore of said housing; central electrode means extending fluid-tightly sealed through said tubular insulating member and having a free end projecting beyond one end portion of said tubular insulating member; a second electrode fixed to said housing and having a portion pposite and spaced from said free end of said central elecrode means to define a spark gap therewith, said one end i0 portion of said tubular insulating member being subjected at the outer surface thereof, upon operation of said spark plug in an internal combustion machine, to contact with hot combustion gases, said outer surface of said one end portion of said tubular insulating member being formed of a material consisting essentially of spinels of aluminum oxide and cobalt oxide and having within a range of between 500 and l000 C. and within a wave length range of between 1 and 7 microns an emissivity of at least 0.7.

2. A spark plug, comprising, in combination, a housing formed with an axial bore therethrough; an elongated tubular insulating member extending fluid-tightly sealed through said bore of said housing; central electrode means extending fluid-tightly sealed through said tubular insulating member and having a free end projecting beyond one end portion of said tubular insulating member; a second electrode fixed to said housing and having a portion op posite and spacedfrom said free end of said central electrode means to define a spark gap therewith, said one end portion of said tubular insulating member being subjected at the outer surface portion thereof, upon operation of said spark plug in an internal combustion machine, to contact with hot combustion gases, said outer surface portion of said one end portion of said tubular insulating member being formed of an intermediate layer of spinels of aluminum oxide and cobalt oxide as a dark material and a cover layer of cerium oxide as a light material, said intermediate layer and said cover layer having within a range of between 500 and 1000 C. and within a wave length range of between 1 and 7 microns an emissivity of at least 0.7.

3. A spark plug, comprising, in combination, a housing formed with an axial bore therethrough; an elongated tubular insulating member extending fluid-tightly sealed through said bore of said housing; central electrode means extending fiuid'tightly sealed through said tubular insulating member and having a free end projecting beyond one end portion of said tubular insulating member; a second electrode fixed to said housing and having a portion opposite and spaced from said free end of said central electrode means to find a spark gap therewith, said one end portion of said tubular insulating member being subjected at the outer surface portion thereof, upon operation of said spark plug in an internal combustion machine, to contact with hot combustion gases, said outer surface portion of said one end portion of said tubular insulating member being formed of a layer of a material including an effective amount of spinels of aluminum oxide and cobalt oxide, said layer having a thickness equal to between about two and three times the absorption coefiicient of said material.

4. A spark plug, comprising, in combination, a housing formed with an axial bore therethrough; an elongated tubular insulating member extending fluid-tightly sealed through said bore of said housing; central electrode means extendin fluid-tightly sealed through said tubular insulating member and having a free end projecting beyond one end portion of said tubular insulating member; a second electrode fixed to said housing and having a portion opposite and spaced from said free end of said central electrode means to define a spark gap therewith, said one end portion of said tubular insulating member being subjected at the outer surface portion, upon operation of said spark plug in an internal combustion machine, to contact with hot combustion gases, said outer surface portion of said one end portion of said tubular insulating member being formed of a layer of a material consisting essentially of spinels of aluminum oxide and cobalt oxide and having within a range of between 500 and 1000 C. and within a wave length range of between 1 and 7 microns an emissivity of at least 0.7, said layer having a thickness of at least 0.02 mm.

5. A spark plug, comprising, in combination, a housing formed with an axial bore therethrough; an elongated tubular insulating member extending fluid-tightly sealed through said bore of said housing; central electrode means extending fluid-tightly sealed through said tubular insula ing member and having a free end projecting beyond one end portion of said tubular insulating member; a second electrode fixed to said housing and having a portion opposite and spaced from said free end of said central electrode means to define a spark gap therewith, said one end portion or" said tubular insulating member being subjected at the outer surface portion thereof, upon operation of said spark plug in an internal combustion machine, to contact with hot combustion gases, said outer surface portion of said one end portion of said tubular insulating member being formed of a layer of a material consisting essentially of spinels of aluminum oxide and cobalt oxide and having within a range of between 500 and 1000 C. and Within a Wave length range of between 1 and 7 microns an emissivity of at least 0.7, said layer having a thickness of between about 0.02 mm. and 0.1 mm.

6. In a spark plug, in combination, first and second electrode means having end portions respectively located adjacent but spaced from each other, said end portions forming a spark gap therebetween; and an insulating member having an end portion located in the vicinity of said spark gap-forming end portions of said electrode means so that upon operation of said spark plug in an internal combustion machine the outer surface of said end portion of said insulating member will be exposed to hot combustion gases, said outer surface of said insulating member consisting of a material including an eifective amount of.

spinels of aluminum oxide and cobalt oxide.

7. In a spark plug, in combination, first and second electrode means having end portions respectively located adjacent but spaced from each other, said end portions forming a spark gap therebetween; and an insulating member having an end portion located in the vicinity of said spark gap-forming end portions of said electrode means so that upon operation of said spark plug in an internal combustion machine the outer surface of said end portion of said insulating member Will be exposed to hot combustion gases, said. outer surface of said insulating member including an efiective amount of spinels of aluminum oxide and cobalt oxide.

References Cited UNITED STATES PATENTS 3,278,785 11/1916 Hauth 3l3137 FOREIGN PATENTS 396,567 8/1932 Great Britain.

DAV ID I. GALVIN, Primary Examiner.

JANIES \V. LAWRENCE, Examiner.

C. BARAFF, Assistant Examiner. 

1. A SPARK PLUG, COMPRISING, IN COMBINATION, A HOUSING FORMED WITH AN AXIAL BORE THERETHROUGH; AN ELONGATED TUBULAR INSULATING MEMBER EXTENDING FLUID-TIGHTLY SEALED THROUGH SAID BORE OF SAID HOUSING; CENTRAL ELECTRODE MEANS EXTENDING FLUID-TIGHTLY SEALED THROUGH SAID TUBULAR INSULATING MEMBER AND HAVING A FREE END PROJECTING BEYOND ONE END PORTION OF SAID TUBULAR INSULATING MEMBER; A SECOND ELECTRODE FIXED TO SAID HOUSING AND HAVING A PORTION OPPOSITE AND SPACED FROM SAID FREE END OF SAID CENTRAL ELECTRODE MEANS TO DEFINE A SPARK GAP THEREWITH, SAID ONE END PORTION OF SAID TUBULAR INSULATING MEMBER BEING SUBJECTED AT THE OUTER SURFACE THEREOF, UPON OPERATION OF SAID SPARK PLUG IN AN INTERNAL COMBUSTION MACHINE, TO CONTACT WITH HOT COMBUSTION GASES, SAID OUTER SURFACE OF SAID ONE END PORTION OF SAID TUBULAR INSULATING MEMBER BEING FORMED OF A MATERIAL CONSISTING ESSENTIALLY OF SPINELS OF ALUMINUM OXIDE AND COBALT OXIDE AND HAVING WITHIN A RANGE OF BETWEEN 500 AND 1000*C. AND WITHIN A WAVE LENGTH RANGE OF BETWEEN 1 AND 7 MICRONS AN EMISSIVITY OF AT LEAST 0.7. 